Gas to gas heat exchangers (GGH) are commonly used in combustion plant desulfurization systems to transfer heat from untreated relatively hot flue gas to treated relatively cooler flue gas. A conventional rotary gas to gas heat exchanger 10, such as that illustrated in FIG. 1 includes a rotor 12 mounted within an interior 14a of a housing 14. The housing 14 defines an untreated flue gas inlet duct 16 and an untreated flue gas outlet duct 18 for a flow represented by arrow 20 of untreated flue gas FG through the heat exchanger 10. The housing 14 further defines a treated flue gas inlet duct 22 and a treated flue gas outlet duct 24 for a flow represented by arrow 26 of treated flue gas TG through the heat exchanger 10. The rotor 12 includes a plurality of radial partitions 28 or diaphragms defining compartments 30 therebetween for element supporting baskets (frames) 32 of heat transfer elements 34. The rotary gas to gas heat exchanger 10 is divided into a treated flue gas sector 38 and an untreated flue gas sector 36 by sector plates 40, which extend across to “cap” open top end 42 and open bottom end 44 of housing 14 to partially enclose rotor 12 within interior 14a of housing 14.
FIG. 2 illustrates an element supporting basket 32 including a few heat transfer elements 34 stacked therein. While only a few heat transfer elements 34 are illustrated in FIG. 2 for purposes of clarity, it will be appreciated that interior 32a of the element supporting basket 32 will typically be filled with multiple heat transfer elements 34. As such, heat transfer elements 34 are closely stacked in a spaced relationship within interior 32a of element supporting basket 32 to form passageways 46 between the heat transfer elements 34 for the flow of untreated flue gas FG or treated flue gas TG therethrough.
Referring to FIGS. 1 and 2, the untreated hot flue gas FG has a flow 20 through the untreated gas sector 36 of the heat exchanger 10 transferring heat to the heat transfer elements 34 on the continuously rotating rotor 12. The heat transfer elements 34 in element supporting baskets 32 rotate about axis 48, illustrated by arrow 50, out of untreated flue gas sector 36 and into the treated flue gas sector 38 of the heat exchanger 10. In treated flue gas sector 38, treated flue gas TG has a flow 26 between the heat transfer elements 34. Treated flue gas TG is thereby heated by heat transfer elements 34. In other forms of heat exchangers 10, the heat transfer elements 34 remain stationary while the untreated flue gas inlet duct 16/untreated flue gas outlet duct 18 and treated flue gas inlet duct 22/treated flue gas outlet duct 24 of housing 14 rotate. For examples of other heat transfer elements 34, reference is made to U.S. Pat. Nos. 2,596,642; 2,940,736; 4,396,058; 4,744,410; 4,553,458; and 5,836,379.
During operation of gas to gas heat exchanger 10, the heat transfer elements 34 in the element supporting baskets 32 accumulate fly ash and substances, such as for example heavy metals, carbon, sulfuric acid, lime, limestone and like substances, onto the surfaces thereof from the flue gas streams. The increasing accumulation of substances on the surfaces of the heat transfer elements 34 in the element supporting baskets 32 causes a corresponding increasing pressure drop over the heat exchanger 10. Removal of accumulated substances from the heat transfer elements 34 in the elemental supporting baskets 32 is typically accomplished using a high pressure water wash. Cleaning or removal of accumulated substances from the surfaces of the heat transfer elements 34 correspondingly eliminates the above-noted pressure drop over the heat exchanger 10. Thus, removal of accumulated substances from surfaces of the heat transfer elements 34, returns heat exchanger 10 flow pressure to pre-substance accumulation levels. However, during the process of cleaning the heat transfer elements 34 to remove accumulated substances therefrom, removed accumulated substances become re-entrained and re-enter the flue gas stream. As such, the re-entrained accumulated substances from the gas to gas heat exchanger 10 enter and are at least in part collected in possibly an associated sea water flue gas desulfurization scrubber (not shown) arranged downstream with respect to the flow of untreated flue gas FG from the gas to gas heat exchanger 10. From the sea water flue gas desulfurization scrubber, the accumulated substances, including for example heavy metals, carbon, sulfur compounds, and like substances, enter an associated sea water treatment plant and potentially discharged to the sea. A solution to reduce or eliminate accumulated substances from the gas to gas heat exchanger 10 becoming re-entrained in the flue gas stream and potentially discharged to the environment is needed.
Thus, there is a need for a gas to gas heat exchanger with heat transfer elements that may be cleaned of accumulated substances to provide decreased pressure drop for a given amount of heat transfer without the dislodged or removed accumulated substances from the gas to gas heat exchanger becoming re-entrained in the flue gas stream and potentially discharged to the environment.